New to XMMS plugin development

I am just trying to start out in developing visualization plugins for 
XMMS, and wondered if anyone would be so kind as to answer a couple of 
simple questions I have.
I have tried to make a simple spectrum analyser, using a 
render_frequency function which takes as it's input (gint16 
freq_data[2][256]).  It all works fine, but when I plot the data, it is 
VERY strongly biased towards the lower array elements, and not a 
realistic frequency spectrum at all.

I also get a very similar form when I use pcm_data[2][512].  As far as I 
was aware, the PCM data, when plotted, should give something like a 
waveform.

I obviously need these basic issues sorted out before I can take this 
any further and would be most grateful for any help

Guy Griffiths